1. Field of the Invention
The invention relates generally to a method for making a current-perpendicular-to-the-plane (CPP) giant magnetoresistance (GMR) sensor that operates with the sense current directed perpendicularly to the planes of the layers making up the sensor stack, and more particularly to method for making a CPP-GMR sensor with a confined-current-path (CCP) for the sense current.
2. Background of the Invention
One type of conventional magnetoresistive sensor used as the read head in magnetic recording disk drives is a sensor based on the giant magnetoresistance (GMR) effect. A GMR sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu). In one type of GMR sensor, called a “spin-valve” (SV), one ferromagnetic layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and the other ferromagnetic layer has its magnetization direction “free” to rotate in the presence of an external magnetic field. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the fixed-layer magnetization is detectable as a change in electrical resistance. The magnetoresistance of the sensor is measured as (ΔR/R), where ΔR is the maximum change in resistance.
In a magnetic recording disk drive read sensor or head, the magnetization of the fixed or pinned layer is generally perpendicular to the plane of the disk, and the magnetization of the free layer is generally parallel to the plane of the disk in the absence of an external magnetic field. When exposed to an external magnetic field from the recorded data on the disk, the free-layer magnetization will rotate, causing a change in electrical resistance. If the sense current flowing through the sensor is directed parallel to the planes of the layers in the sensor stack, the sensor is referred to as a current-in-the-plane (CIP) sensor, while if the sense current is directed perpendicular to the planes of the layers in the sensor stack, it is referred to as current-perpendicular-to-the-plane (CPP) sensor.
In CPP-GMR sensors, because the sense current flows perpendicular to all the layers in the sensor stack, the resistance of the active region (the free layer, spacer layer and pinned layer in a SV) is a relatively small part of the total resistance of the sensor. Due to its high resistivity, the antiferromagnetic layer can account for more than 90% of the total stack resistance. It is thus desirable to increase the resistance of the active region without significantly increasing the total resistance. One approach to achieving this is sometimes called a confined-current-path (CCP) sensor, wherein the sense current is forced to pass though only a portion of the area of the sensor stack. One type of CCP CPP-GMR sensor has a CCP layer in the form of a partially-oxidized nano-oxide layer (NOL) in the active region, typically in the conductive spacer layer. The sense current is confined to flow only though the conductive non-oxidized areas of the NOL. The NOL thus increases the resistance of the active region (which also contributes to the ΔR) preferentially over that of the inactive regions (which only contribute to R) and therefore increases the overall magnetoresistance (ΔR/R) of the sensor. CPP-GMR sensors with NOLs are described by Oshima et al., “Current-perpendicular spin valves with partially oxidized magnetic layers for ultrahigh-density magnetic recording”, IEEE Transactions on Magnetics, Vol. 39, No. 5, September 2003, pp. 2377-2380; and by Fukuzawa, et al., “MR Enhancement by NOL Current-Confined-Path Structures in CPP Spin Valves”, IEEE Transactions on Magnetics, Vol. 40, No. 4, July 2004, pp. 2236-2238.
Because the formation of the conductive paths in the NOL is by oxidation and annealing of a very thin layer, the number and size of the conductive non-oxidized areas depends on the material properties, layer thickness, oxidation time, and anneal conditions. As a result it is difficult to reliably manufacture large quantities of CCP CPP-GMR sensors with NOLs with predictable values of R and ΔR/R. In addition, the conductive non-oxidized areas of the NOL are generally randomly distributed across the entire plane of the spacer layer. Fujiwara, et al., “Magnetic and Transport Properties of GMR/Spin-Valves and Their Components”, University of Alabama Materials for Information Technology (MINT) Spring Review, April, 2002, proposes a CCP CPP-GMR sensor wherein generally evenly distributed pin holes that function as confined current paths can be lithographically formed in the sensor stack.
It has been demonstrated that some cage-shaped proteins, such as ferritin or apo-ferritin (in the following just called ferritin), which feature both biomineralization in their inner cavity and the ability to self-assemble into nanostructures, can be used to build inorganic nanostructures for use in semiconductor devices. Ferritin is a supramolecular protein having a generally spherical shell with inner and outer diameters of 7 nm and 12 nm, respectively. There are narrow channels connecting the outside of the ferritin molecule with its inner cavity, through which ions can enter and mineralize into inorganic particles which form a core within the ferritin shell. Ferritin proteins have been prepared with inorganic Co-oxide, Cr-oxide, Fe-oxide, and ferrihydrite cores. It has been shown that other even smaller cage-shaped proteins like Lis-Dps with inner and outer diameters of 4.5 nm and 9 nm, respectively, can be used for the same task. It has been demonstrated that ferritin molecules can be modified to have peptides with specific affinity to a target material, such as a carbon (C) or titanium (Ti) surface. These modified ferritin molecules are attracted to the carbon or titanium surface and to each other to self-assemble into a two-dimensional hexagonal close-packed array of high density and uniformity. After the proteins are attached to the target surface the ferritin molecules can be dissolved by heat or an ultraviolet (UV)/ozone process, leaving behind a regularly-spaced array of the inorganic cores, which can be used as an etching mask. The above-described features of ferritin, the processes for making them, depositing them into self-assembled arrays and dissolving them are described in numerous publications, including the following:    Yamashita, “Fabrication of a two-dimensional array of nano-particles using ferritin molecule”, Thin Solid Films, Volume 393, Issues 1-2, 1 Aug. 2001, Pages 12-18;    Kubota et al., “A 7-nm nanocolumn structure fabricated by using a ferritin iron-core mask and low-energy Cl neutral beams”, Appl. Phys. Lett. 84, 1555 (2004);    Kubota, et al., “Low-damage fabrication of high aspect nanocolumns by using neutral beams and ferritin-iron-core mask”, Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, May 2007, Volume 25, Issue 3, pp. 760-766;    T. Matsui et. al., “Realizing a Two-Dimensional Ordered Array of Ferritin Molecules Directly on a Solid Surface Utilizing Carbonaceous Material Affinity Peptites”, Langmuir 2007, 23, 1615-1618; and    T. Matsui et al., “Direct Production of a Two-Dimensional Ordered Array of Ferritin-Nanoparticles on a Silicon Substrate”, Japanese Journal of Applied Physics, Volume 46, Issue 28, pp. L713-L715 (2007).
What is needed is a CCP CPP-GMR sensor that can be manufactured using ferritin molecules to control the size and location of conductive areas in a CCP layer in the sensor stack.